The present invention is primarily concerned with the problem of and the consequences associated with "popping" of sewer lids by excess fluid or pressure within a drainage system, a situation which is not infrequent in areas which experience extreme weather conditions. One serious consequence is that removal of the cover may be sufficiently forcible as to cause injuries to persons or damage to property in the vicinity of the manhole. Another consequence is that the cover may be carried away with the stream which issues from the manhole opening, thereby requiring retrieval and replacement of the cover. More importantly, this conditions results in an open manhole which, once the excess pressure has been relieved, may present an invitation to children or, as has been known to happen, may actually suck a person into the manhole as water retreats back into the manhole.
Canadian Pat. No. 303 918 granted on Sept. 16, 1930 proposes a solution to the aforementioned problem and, in particular, a manhole cover having a depending skirt having slots extending upwardly from the bottom of the skirt in a tortuous fashion and adapted to receive pins extending inwardly from the manhole opening. Thus, should the water level within the manhole rise up against underside of the cover, the pins and slots cooperate to permit the cover and skirt to rise until the pins reach a horizontal portion of the slots at which point the pins and slots cooperate to hold the cover in an open position. Accordingly, excess water is permitted to escape through openings in the skirt. This arrangement does not entirely illeviate the afore-discussed problems. Firstly, it is conceivable that the pins could be freed from the slots, and the cover thus dislodged, if the water pressure is sufficiently high and applied abruptly, such as by an explosion within the manhole. Secondly, since the device is designed to hold the cover in an open position, it is still necessary to manually close the cover. Thirdly, the complex slot design is relatively difficult and, therefore, expensive to manufacture.